Multipath is an ever-present problem for all RF communications and positioning systems. It causes degraded signal strengths and low data bandwidths for communication systems, and inaccurate range measurements for positioning systems. In particular, multipath in an indoor environment is very severe, with signals being reflected from most objects including walls, ceilings, floors, furniture, and people.
A number of diversity methods have been developed to mitigate the effects of multipath in communication-based systems. They include spatial, frequency, and polarization diversity. These traditional methods are designed to minimize signal cancellation which is caused by the direct and reflected signals interacting in a destructive manner. These prior art systems generally either: (a) deploy a plurality of spatially distributed receive antenna elements (spatial diversity), which are configured with a control means to continuously select the antenna element with the highest signal strength, or (b) deploy a plurality of receive antenna elements which have unique polarization (polarization diversity), which are configured with a control means to continuously select the antenna element with highest signal strength or (c) deploy a single receive antenna element configured to receive a plurality of frequencies transmitting identical information (frequency diversity), and configure a control means to continuously select the frequency with the highest signal strength. The antenna element or frequency with the highest received signal strength is then used to demodulate the communications data. These prior art diversity systems do not discriminate in any way between direct and reflected signals. A strong reflected signal with good signal strength will be accepted over a weaker direct signal. However, for positioning systems to function correctly it is vital that the direct path signal is measured, even though it may not necessarily be the signal with the highest received signal strength. Therefore, traditional prior-art communications-based diversity methods are not suited for the mitigation of multipath in positioning systems.
Traditional multipath mitigation methods for positioning systems fall into five broad categories, as described below:                (1) improved modulation techniques;        (2) improved receiver correlation techniques;        (3) multipath limiting antennas;        (4) over-determined position solutions; and        (5) Receiver Autonomous Integrity Monitoring (RAIM).        
1) Improved modulation techniques for multipath mitigation generally relate to increased chipping rates of Code Division Multiple Access (CDMA) pseudorandom number (PRN) codes. As chipping rates are increased multipath correlation is decreased. However, RF spectrum usage, receiver power consumption and receiver complexity are also increased.
2) Improved receiver correlation techniques achieve multipath mitigation in the receiver correlators by making the autocorrelation tracking function more resilient to multipath perturbations. One of the more prominent techniques in this category is the so-called narrow correlator, whereby early and late correlator tracking arms are reduced from the conventional one-half chip spacing down to one-tenth chip spacing. This narrower spacing is free of long delay multipath, however is still susceptible to short delay multipath. The narrow correlator technique also requires extended receiver bandwidth to improve the sharpness of the autocorrelation function, which increases receiver power consumption and complexity.
3) Multipath-limiting antennas shape the receive and/or transmit antenna gain pattern to reduce the strength of reflected, off-axis signals. The most common form of this antenna is the so-called choke ring antenna, generally used in GPS applications for mitigating ground reflections. Multipath-limiting antenna methods orient the beam pattern of the antenna in one direction and, as such, have limited application in high multipath environments such as indoors, where signals reflect from many directions.
4) Over-determined position solutions use more transmitters than required to form a position solution. This improves position accuracy by decreasing the significance of multipath corrupted positioning signals in the position solution. An added advantage is the improved geometry provided by more geometrically diverse transmitters. However, for this method to be effective, the majority of positioning signals must be non-corrupt at any given time. This is generally not the case in high multipath environments.
5) Receiver Autonomous Integrity Monitoring (RAIM) is an algorithm employed by position receivers to check the integrity of received positioning signals, and hence eliminate outlier measurements from position solutions. In its simplest form a RAIM algorithm observes range residuals from different combinations of transmitter positioning signals to determine multipath-corrupted outlier ranges. Transmitters that are associated with large range residuals are deemed multipath corrupted and are eliminated from the position solution. Different transmitter combinations are achieved by using redundant positioning signal measurements from additional transmitters. Therefore, the RAIM method requires a significant number of redundant transmitters to provide an effective multipath mitigation tool, which in many cases is highly impractical. Furthermore, if the RAIM algorithm deems a positioning signal to be multipath corrupted, the transmitter is eliminated from the position solution, which further reduces geometric diversity.
Geometry-Free Positioning Signals
Geometry-free positioning signals are defined as unique positioning signals that are transmitted on individual carrier frequencies from the same location, such that the unit vector and geometric range of each unique positioning signal is identical with respect to an observing receiver. This requires the transmission of a plurality of frequencies through the same phase centre of the same physical antenna array, which in practice becomes increasingly difficult as the frequencies become dispersed. Furthermore, accurate chronological synchronization of geometry-free positioning signals is also difficult, as the group delay and line biases of the electronic components within each frequencies transmission path independently vary with temperature and voltage. Geometry-Free positioning signals are generally used for the determination of so called “cycle slips” in carrier-based positioning systems. A cycle slip is a sudden integer, or half cycle jump in the carrier phase observable of a positioning signal, caused by a loss of lock of the observing receiver carrier tracking loop (generally a phase-lock-loop). In single frequency positioning systems it has proven difficult to accurately and reliably detect and repair cycle slips. Traditional cycle slip detection methods have utilized linear combinations of geometry-free observables, typically between the GPS L1 and L2 carrier frequencies. These geometry-free methods detect cycle slips by observing discontinuities in the Integrated Carrier Phase (ICP) time series of the geometry-free positioning signals. However, these prior art methods do not consider the coherence of geometry-free range measurements in determining multipath corruption.
Prior art systems (a) cannot distinguish between direct and multipath corrupted positioning signals transmitted from substantially the same location, (b) cannot provide multipath mitigation without redundant geometrically-diverse transmitters, (c) cannot provide a diversity system free from the encumbrance of time varying group delay, and (d) cannot improve multipath mitigation without increased transmission bandwidths, increased receiver power consumption, and increased receiver complexity. A system that can provide precise range measurements without any of these constraints is highly desirable.
Definition of Terms
Range vector—A range vector is a vector that defines the spatial relationship of two points in space by defining the direction and distance from the first point to the second point. For example, given two points in space, p1 and p2, with associated position vectors  and , the range vector  from p1 to p2 is defined as vector subtraction of the position vectors, such that =−.  defines the direction and distance from point p1 to p2.
Geometric range—Geometric range is the scalar distance between two points in space. For example, the geometric range of a range vector  is defined as ||.
Unit vector—A unit vector is a vector with a magnitude of unity. Unit vectors are used to define direction. For example, the direction of a range vector, , can be represented by a unit vector, {circumflex over (r)}, that has the same direction as  and unity magnitude.
Diversity—Diversity, as set forth in the present invention, is the transmission and/or reception of radio signals which exhibit unique radio reflections in radio reflective (multipath) environments. Diversity generally is accomplished using either spatial, frequency, or polarization means.
Diverse radio links—Diverse radio links, or radio link diversity, are radio signals which employ diversity methods.
Transmit cluster—A plurality of unique positioning signals which are each synchronously transmitted from substantially the same location, whilst retaining radio link diversity, are known as a ‘transmit cluster’.
Receive cluster—A plurality of discrete positioning signals which are each synchronously received at substantially the same receive location, whilst retaining radio link diversity, are known as a ‘receive cluster’.
Substantially coherent positioning signals—Substantially coherent positioning signals are positioning signals that when received and interpreted by an observing receiver produce measurements that are substantially similar.
Positioning-Unit Device—A Positioning-Unit Device is a form of positioning transmitter, which transmits positioning signals.
Observing receiver—An observing receiver is a receiver that receives and interprets positioning signals.